Boost referencing a Holley carb

[Stoneshrink]

I've got the book, but I'm still not sure why boost referencing works. Anyone care to enlighten me?

[squirrel]

Re: Boost referencing a Holley carb

Beats me, I don't use it, haven't burned a piston yet, lotsa miles with moderate boost.

[Dynoroom]

Re: Boost referencing a Holley carb

I'm going to try to make this shot (ya I know) the reason you would boost reference your carb is because when it sits on top of a blower or on the inlet side of a turbo (draw through) you can pull a vacuum on the carb at full load/boost that would make the power valve think it is running at idle causing a lean condition.
Hope this helps

[squirrel]

Re: Boost referencing a Holley carb

Micheal, do you think that having enough carb cfm could eliminate the need for boost referencing?

[Dynoroom]

Re: Boost referencing a Holley carb

Micheal, do you think that having enough carb cfm could eliminate the need for boost referencing?

The easy way to tell is put a vacuum gauge under the carb. If it's pulling a vacuum at WOT you are asking for trouble.

[JeffMcKC]

Re: Boost referencing a Holley carb

Even a well tuned N/A motor will pull some Vac at WOT when it gets up in RPM

[Dynoroom]

Re: Boost referencing a Holley carb

Even a well tuned N/A motor will pull some Vac at WOT when it gets up in RPM

This is true but it better be less than 1" (unless it a restricted engine) a turbo or blower can "suck" as much as 10" Hg. and close a power valve thus leaning out the engine.
I stated in my other post I was going to short and easy...

[squirrel]

Re: Boost referencing a Holley carb

so would it be ok if the vacuum above the blower is less than the power valve closing point?

[Dynoroom]

Re: Boost referencing a Holley carb

so would it be ok if the vacuum above the blower is less than the power valve closing point?

Yes, that's why it's not always a problem. But I'd want it to be at least 3-4" difference, say a 6.5 P/V with 2.5" of vacuum under the carbs.

[Eric68]

Re: Boost referencing a Holley carb

To boost reference a power valve I would recommend referring to Super Tuning and Modifying Holley Carburetors. Performance Street and Off-road Applications by Dave Emanuel pages 67 - 69.

It's hard to describe without pictures, but the idea is basically this -- block the factory power valve passage in the base plate, and drill a new hole in the throttle body so the vacuum cavity behind the power valve gets it signal from a source under the blower. There are a couple ways to do this . . .

[ulrace]

Re: Boost referencing a Holley carb

My question would be why bother? I have had really good luck with race stuff with the power valve blocked. Just about ten numbers on the jet richer is starting point. Never tried it on the street but was planning to on 671 blown 392.. 2 850 holleys... willing to negotiate..I know it's a popular (and expensive) way to go with the boost referencing hardware.
bob

[BillBallinger Sr]

Re: Boost referencing a Holley carb

My question would be why bother? I have had really good luck with race stuff with the power valve blocked. Just about ten numbers on the jet richer is starting point. Never tried it on the street but was planning to on 671 blown 392.. 2 850 holleys... willing to negotiate..I know it's a popular (and expensive) way to go with the boost referencing hardware.
bob

I think you are cheating yourself out a cleanness of mixture without the PVs. Referencing boost is straight forward, you are allowing for a change of factor perceived by the carbs atmospheric pressure between vacuum and boost that is caused by the supercharger. It is a decision to be made in designing your system, going positive under the blower and losing 10 jet sizes in your mixture is a bad thing, and running around at low speed 10 sizes too fat isn't much better if you are washing down the walls and rings. Dave Emannuel's book is a good one. The principles are great, and will make certain things show their potential that may not always be self evident.

Just my thoughts.

[ulrace]

Re: Boost referencing a Holley carb

Thanks Bill1 I guess race stuff at WOT is a little different. I ran this setup mostly on hydroplanes which were nearly aleayd on. Still came off the corner good without power valves and ran lean enough on the top end. I will look into the setup DF used on the procharger setup. 671 will be sucking air through the carbs though. Wouldn't the signal be valid? Curious as this is to be streetable blown engine.

[Eric68]

Re: Boost referencing a Holley carb

What you are trying to accomplish with a boost referenced power valve is getting the power valve to operate correctly with a blower. The power valve needs to see the same vacuum (or pressure) that the engine is seeing (after the blower) so it opens correctly -- the signal above/before the blower is not the same.

You could probably get away without boost referencing with a small blower and a big carburetor because the blower isn't strong enough to pull a vacuum on the bottom of the carb at WOT, but if that blower ever manages to pull enough vacuum to close the power valve at WOT you will go lean and hurt a piston. If you just jet up and remove the power valves then you will be rich at cruise on the street and tend to foul plugs.

A procharger setup is completely different because it is blow through instead of a draw through like a 6-71. With a blow through the power valve will see pressure as soon as the engine goes to WOT so it will still function normally. The only thing to do with a power valve on a blow through setup is to remove the blowout protection so the boost pressure doesn't make the little check ball stick.

Hope that helps

[squirrel]

Re: Boost referencing a Holley carb

Something no one has mentioned yet, is that you can run progressive throttle linkage, and play some interesting games with jetting. Although years ago someone told me I'd melt pistons running progressive linkage, because "the fuel goes straight down" and only the primary end of the engine would get fuel. I've never noticed any difference in how the plugs look, and 75k miles later the pistons still work.